The objective of this proposal is to study the genetics of murine sarcoma virus (MuSV) and its relatedness to murine leukemia virus (MuLV) using two MuSV cell lines, namely MuSV 349 and 15F, as well as temperature-sensitive mutants of MuLV and MuSV. In order to achieve this objective we propose to investigate 1) the role of endogenous virus in the replication of defective MuSV, 2) the role of helper virus (both endogenous and exogenous) in the expression of transformation, and 3) the genetic information MuSV has in common with MuLV.